Broadband communication networks are used in a variety of high speed application services such as internet access, video conferencing, video on demand, and interactive TV. Although fiber optic cable is the preferred transmission media for such high data rate services, the lack of fiber optic cables in existing networks and the prohibitive costs of installing such optical networks have led telephone companies around the world to include existing twisted-pair loops in their next generation broadband access networks. Because current telephone wiring connections were not designed to support high speed data communications, technologies were developed to increase the transmission capabilities of existing telephone wiring.
One technology for providing such high data rate services on existing twisted-pair connections is Asymmetrical Digital Subscriber Line (ADSL). ADSL transfers data over the higher frequencies in the twisted-pair copper wires that currently connect most homes and businesses. ADSL accomplishes this by increasing the transmission capabilities of the current telephone wiring connections. Thus, ADSL technology enables data to be exchanged over the twisted-pair copper wires at much higher speeds than conventional modems and analogue lines.
Several standards have been adopted and published by the International Telecommuncation Union (hereinafter “ITU”) to standardize use and performance of ADSL systems. For example, the ADSL1 standard (G.992.1) uses the discrete multi-tone (DMT) modulation technology. DMT modulation divides the available bandwidth of communications channels into multiple carriers, also referred to as bins or sub-channels, and employs the multiple carriers for both upstream and downstream communication. Each carrier is allocated a number of bits to send with each transmission. Frequency-division-multiplexing is often used, where upstream and downstream communication use carriers in different frequency bands. Thus, DMT modulation maximizes the available channel capacity by using a large number of carriers rather than a single carrier, and thereby optimizes performance of the transmission.
Communication between a central office and a customer premise typically travels in both a downstream direction and an upstream direction. In the downstream direction, it is state of the art to cut back (i.e. reduce) power. However, in the upstream direction, the power level of the transmission is fixed in the state of the art. Currently, in ADSL transceivers compliant with the ADSL1 standard (ITU G.992.1 and G.992.2 standards), there is no way to reduce the transmit power of the signals generated by the CPE when operating on short distances. While a connection between the central office side and the subscriber side is established, the transceivers of both modems continue to monitor the changing signal-to-noise ratios on the line and swap bits from one carrier to another to maintain system performance.
Some disadvantages in using a fixed power level in the upstream direction are power consumption and crosstalk. A central office has to deal with modems both near and far. Modems which are too near often result in power transmissions which are unnecessarily high. Modems which are far have high power attentuation. The result is a huge difference in the dynamic power range. Cross-talk affects all types of DSL transmission. If the effect of signal crosstalk in the loop plant is not reduced, then the upstream signal power variations seen at the central office may be so high that the crosstalk coupling degrades the upstream performance of the long lines.
Recommendation G.992.3 published by the International Telecommunication Union (ITU) (also referred to as ADSL2 or G.992.3 standard), which is incorporated herein by reference, provides an upstream power backoff procedure by introducing a maximum received power parameter: MAXRXPWR. This parameter defines the maximum power that the Central Office Equipment (hereinafter “CO”) should receive from the Customer Premise Equipment (hereinafter “CPE”). During the Channel discovery phase of the initialization sequence, the CO measures its received signal power and computes the amount of power backoff that should be applied on the upstream signal. This backoff value is communicated to the CPE during this same phase of the initialization sequence in the C-MSG-PCB message. Thus, the solution in G.992.3 ensures that the configured maximum receive power will never be exceeded. The desirable result is the reduction of the effect of signal crosstalk in the loop plant.
While the ADSL2 standard (G.992.3) provides one solution to the upstream power backoff procedure, it is not widely useable since many existing CPE modems do not currently support the ADSL2 (G.992.3) standard. Devices which operate in accordance with ADSL1 (ITU recommendations G.992.1 or G.992.2 and which are incorporated herein by reference) do not support a MAXRXPWR control parameter and hence are not able to apply a power backoff to modify the upstream power signal. Moreover, unlike the ADSL2 standard, the signal-to-noise ratio (SNR) that would result from applying such a power backoff in an ADSL1 compliant device cannot be directly measured. Indeed, there is no provision in the ADSL1 standard to offer a specific known signal in which the transmit power is reduced that can be used by the receiver to measure the resulting SNR. Therefore, a solution is still needed to provide upstream power cutoff in ADSL transceivers compliant with the ADSL1 standards.
It is an object of the present invention to solve the aforementioned problems and more specifically, to solve the aforementioned problems by providing a communication system which reduces the upstream power level in an ADSL communication system.